This application is based on a provisional application Ser. No. 60/397,188 filed Jul. 19, 2002. The present invention relates to a method for stimulating the completion of hydrocarbon wells in subterranean formation and more particularly, to provide injection wells with an essentially uniform injection profile.
In the recovery of hydrocarbons from subterranean formations, it is a common practice to stimulate the production of a productive wellbore by providing secondary wells in which a fluid, typically water or brine, is injected to increase the formation pressure and displace the hydrocarbons towards the production wells. In this type of recovery operations, it is key to be able to inject large quantity of fluids with a uniform profile to maximize the efficiency of the sweeping up process.
Most of the time, in sand environment, such injection wells are completed open-hole, meaning that in the injection area, the well is not provided with a casing and not perforated. In unconsolidated formations however, sand control measures are implemented to prevent wellbore collapse. Common practice for controlling sand displacement includes uses of standalone screens so that the well collapses around the screen but remains open inside the screen, slotted liners (including expandable screens) or placement of gravel pack to hold formation sand in place. The gravel pack is typically deposited around a perforated liner or screen. The gravel pack filters the sand while still allowing formations fluid to flow through the gravel, the screen and a production pipe.
Although targeted flow rates may sometimes be achieved without any chemical cleanup, it has long been recognized that filter cake cleanup is suitable to improve the injection efficiency. The filter cake is formed during the drilling operation by the build-up of the solid phases present in the drilling fluid and filtered by the formation as the drilling fluid tends to percolate into the formation since an overbalance pressure is often applied which causes fluid loss from the wellbore into the reservoir rock. The filter is actually suitable during the well drilling step since it prevents undesirable leakage of drilling fluids but this impervious barrier is detrimental to suitable migration of fluids such as injection fluids.
The solids constituting the filter cake consist of solid additives such as starches or other type of viscosifying agents, bridging agents such as sized calcium carbonate particles and other drilled solids picked up during the drilling phase such as fines, silt or sand particles.
A treatment with a clean-up fluid followed by stage in which the well is allowed to produce during a certain period is typically desirable to remove the filter cake and maximize infectivity. However, this solution is often not practicable either due to limited storage capacity on the rig or requirement for artificial lift due to low reservoir pressure. Therefore, treatments have to be carried out to remove the filter cakes. Another possibility is to inject a fluid at such a pressure and injection rate that it exceeds the fracturing pressure to bypass the filtercake damage. However, this solution is usually not desirable from a sweep efficiency standpoint or due to premature breakthrough or may not be feasible, e.g. due to very high fracturing pressures or pump limitations.
Numerous chemicals and methods using these chemicals have been proposed to remove filter cakes. These include: aqueous solution with oxidiser present, acid wash solution (mineral acids such as hydrochloric acids or organic acids such as formic and acetic acid), combinations of acid and oxidiser, and aqueous solution with enzymes. Reference is made for instance to U.S. Pat. Nos. 5,126,051 and 5,165,477 that disclose the admixture of enzymes to a drilling mud to promote the enzymatic decomposition of the polymeric organic components of the filter cakes. U.S. Pat. No. 5,881,813 discloses treating injection wells with an enzymatic clean-up fluid to degrade the polymeric viscosifiers and then, perform and acidizing treatment to remove the non-polymeric solids. U.S. Pat. No. 6,263,967 discloses the use of an aqueous wash composition for cleaning up drilling and completion fluid filtercakes comprising water and an effective amount of cationic salts of polyaminocarboxylic acids (such as a di-cation salt of (ethylenedinitrilo) tetraacetic acid), at neutral or acidic pH. The solution may further comprise an oxidizer or enzymes.
For the majority of the recent drilling fluids used in reservoir sections, the acids do not substantially break-down the polymeric component and in fact are used for removal of the calcium carbonate. The oxidizers and enzymes attack the polymers whilst the acids mainly attack the carbonate and polymers. This facilitates the back-production of the filtercake through the screens and limits damage of the completion.
In injection wells however, it has been found that the enzymatic (or oxidizing) treatment followed or combined with an acid treatment is not really satisfying for injection rates. In particular, this type of treatment does not provide a uniform filter cake removal, including drill solids as well as the bridging agents and polymers though this uniform removal is highly suitable to ensure uniform injection profile and maximize infectivity index. This cleaning process is time consuming, costly and often of poor efficiency. It is therefore desirable to provide a new way of completing injection wellbores.